In ink-jet technology, the quality of high-resolution images is a function of the ink-jet ink used to produce an image, the printer and the print medium upon which the image is printed. In ink-jet printing, droplets of ink are ejected from a printhead in response to electrical signals generated by a microprocessor and are deposited on a print medium, such as paper or polymeric substrates, to form the desired image.
Colorant and inks for use in ink-jet printing should provide good print quality, reliability performance, and environmental robustness, when used individually as well as in combination with other dyes and inks. A single ink-jet colorant and/or ink that has good chroma, gamut, hue angle, and environmental robustness (e.g., air-fastness, light-fastness, water-fastness) is not always optimal for use with other colorants and/or ingredients of that ink, or other inks with which it is used in combination. In other words, not only does an individual colorant and/or ink (e.g., cyan, magenta, or yellow ink), have to independently have acceptable color qualities, but it should also work well when used as part of a dye and/or ink set. Additionally, although many inks are known which possess one or more desirable properties, a drawback of many inks is that improvement in one property often results in degradation in another desirable property.
Three-ink dye based printing systems comprising cyan, magenta and yellow inks commonly suffer from high lightness (L*min) in the composite black regions of images, resulting in poor image quality. The issue is often compounded by hazing of prints on special porous photo media, sometimes referred to as “stacked haze.” This type of haze occurs after printing, as a result of stacking of printed images. All of the stacked images, other than the uppermost one which is exposed to air and dries quickly, display a hazy film on the printed images. The hazing effect can increase the lightness of the black regions, thus reducing the sharpness of images. Use of a dye-based black ink together with the three-ink composite black, to make a four-ink system, can help to improve the lightness of the black regions. However, the choice of the colorant (i.e., the selected chemical dye compounds) determines the lightness, permanence to light and ozone as well as the extent of observed haze on the images. While some black dyes, or a combination of black dyes, as colorants in black inks have been shown to reduce the lightness of black regions in images, they do not address the issue of hazing.
There is continuing interest in developing colorants and black inks that improve image quality while maintaining permanence, especially on porous print media.